1. Field of the Invention
The present invention relates to fine wire bonders and more specifically to the apparatus for feeding and tearing fine wires being bonded by a wedge bonding tool of a very high speed automatic wedge wire bonder.
2. Description of the Prior Art
Wedge bonders are known in the prior art. Modern wedge bonders are provided with wire clamps which clamp and pull on the wire to tear or break the wire behind the second or last wire bond while the bonding tool is held on the bond position. After the wire is broken behind the foot of the wedge bonding tool, the bonding tool may be raised away from the work piece and the wire clamp is moved forward to push or feed a length of wire under the working face of the wedge bonding tool. Thus, the wedge bonding tool apparatus is prepared for a new first bond.
Various types of actuators have been employed to open and close prior art wire clamps and to move the wire clamps relative to the wedge bonding tool. Manually operated wedge wire bonders have heretofore employed cams which actuate wire clamps to feed and to tear the wires being bonded by a wedge bonding tool. Our U.S. Pat. No. 3,543,988 issued Dec. 1, 1970 shows and describes a high speed ultrasonic tailless wedge wire bonder identified as Kulicke & Soffa Model 484. The main cam bank of this wedge bonder was provided with cams which actuated cam followers on levers which actuated push pins which in turn actuated the wire clamp of the tailless wedge bonder. The push pins in this device were extended through hollow transducer pivot screws. Cams of the type described in the aforementioned patent are actuated by motors which complete a single machine cycle in slightly under 1 second. The actuating cams are capable of imparting sufficient force to the moving mechanism to drive or move parts having large masses. Further, the profiles on the drive cams are arranged such that little or no shock or vibration motion is imparted to the bonding head or the wire bonding tool.
Automatic wedge wire bonders which employ servomotor drives are capable of completing a machine cycle two or three times faster than the prior art cam driven wedge wire bonders. It was discovered that high speed means were required to actuate the wire clamp formerly actuated by cams. Attempts to substitute heavy solenoids in place of cams was not possible because the high speed automatic wire bonder required that the bonding head be rotatable. Attempts to mount light weight high energy solenoids on the bonding tool holder for energizing and actuating the wire clamp created undesirable shock and vibrations. High speed solenoids are actuated in only several milliseconds which creats a large shock force.
It was found that the actuation of high speed solenoids generated an action and reaction inertia effect on the bonding tool holder. When the solenoid plunger of the high energy solenoid reached its limit of travel, the mass of the plunger would suddenly stop and created a second and larger shock effect on the bonding tool holder. Efforts to mount the solenoids near the wire clamp created shock forces which added to the bonding tool force and created vibrations whose settling time extends up to and includes 70 milliseconds. It was found that high speed solenoids when attached to any part of a rotatable wedge bonding head generates some inertia and shock force when actuated. These forces are transmitted throughout the bonding head which supports the bonding tool. Since the bonding tool of a automatic high speed wedge bonder is preferably an ultrasonic transducer the horn or bonding tool holder of the transducer is shaped liked a tapered cantilever beam. Any high impact shock force which creates an undamped vibration having harmonic frequencies will impart ringing to the transducer. If the solenoid is mounted on the bonding head housing or the bonding tool mounting block in a manner which allows the inertia and shock forces to generate a moment force around the bonding tool, some of the moment force is imparted to the working face of the bonding tool so as to change or modify the bonding force applied during a bonding operation.
It was discovered that high speed solenoids which are adapted to operate in under 5 milliseconds are capable of producing ringing vibrations which continue for up to 70 milliseconds. Such vibrations require that the bonding tool be held steady until the vibrations stop to prevent improper bonds. Thus, much of the time saved by employing fast high energy solenoids may be lost due to the aforementioned vibrations.
The greater problem arises when the inertia or impact force of the solenoid applies an appreciable bonding force to the bonding tool during a bonding operation. If the additive forces are high enough they can cause the wire at the bond to be mashed flat. Over squashed or mashed flat bonds are weakened by the reduction in the cross-section of the wire extending from the bond. Such bonds are not acceptable on most devices that are sold commercially. It is not economical to attempt to remove the oversquashed bonds and to rework the semiconductor device. Further, it is not desirable to have to pull test the bonds to determine if they have been oversquashed and been reduced in strength. Semiconductor devices having bonds of widely varying bonding strength are not saleable as commercial products.
It is a primary object of the present invention to provide a high speed solenoid actuated wire feed and tearing apparatus.
It is another object of the present invention to provide a low mass, high speed solenoid for actuating wire clamps employed on rotatable bonding head high speed automatic wedge wire bonders.
It is another object of the present invention to provide a novel solenoid actuated mechanism for positioning a wire clamp in three distinct positions employing a single high speed solenoid.
It is another object of the present invention to provide a novel bonding head having a unique housing and bonding tool mounting block structure for supporting a pair of high speed solenoids.
It is a general object of the present invention to provide a light weight bonding head having a low mass housing and transducer mounting block capable of supporting high speed solenoids without creating ringing in a bonding tool transducer when the solenoids are actuated.
It is a further object of the present invention to provide a novel pivotable transducer mounting block for supporting a wire clamp of a high speed automatic wedge bonder.
According to these and other objects to be discussed hereinafter, there is provided a rotatable bonding head having a housing comprising two parallel and depending arms. Each of the depending arms is provided with a hollow cylindrical transducer pivot screw which extend through the depending arms and are actually aligned so as to pivotally support a transducer mounting block. Solenoid actuated push pins extend through the center of the hollow cylindrical transducer pivot screws and are adapted to actuate levers for articulating the wire clamp and for opening and closing the wire clamp to feed and tear a fine wire being bonded by a wedge bonding tool. The solenoids are mounted on the housing of the bonding head and the actuating levers are mounted on the transducer mounting block so that the inertia and impact forces created by actuation of the high speed solenoids are imparted into the rigid housing having a high mass so as to damp the impact vibrations before they are imparted to the actuating levers and the transducer mounting blocks so as to impart vibration of forces to the bonding tool.